∵（a-b)-(b-a)=(a-b)+(a-b)∴(a-b)(b-c)-(b-a)(c-b)xy=(a-b)(b-c)+(a-b)(b-c)(-xy)=(a-b)(b-c)(1-xy)
=(a-b)(b-c)-(a-b)(b-c)xy=(a-b)(b-c)(1-xy)
